The present invention relates to the use of heat exchangers when transporting unstable fluids.
Pipelines for transporting viscous media, such as for example hot-melting plastics (“hot melts”) are known e.g. from US 2009/0321975. A plant is presented therein, in which a viscous polymer is conveyed to an extruder. The polymer melt is cooled in this case by means of a heat exchanger.
A reactor for continuously carrying out a polymerisation reaction in highly viscous media is described in EP 096 201 A1. It was found that the energy expenditure during mixing and heat transfer at high viscosity is so large that chemical reactions can be influenced during the polymerisation. Thus, means for dissipating the reaction heat and limited mixing times for homogenisation are the aim. To this end, the reactor is perfused with a coolant and static mixing components are provided in the interior. Static mixing components are wide-spread and reference is therefore made to the disclosure of U.S. Pat. No. 7,841,765 or WO 2009/000642.
However, it may not only be polymerisation reactions that require dissipation of the reaction heat. Dissolution processes, intermediate storage of the thermally decomposable polymers and polymer solutions also require an actively controlled temperature management, so that the desired products have a corresponding quality on the one hand and on the other hand, the above listed method steps can also be managed safely with respect to temperature. Recently, many efforts have been undertaken so that biodegradable products, such as cellulose fibres, can be produced from spinning solutions on an industrial scale. Spinning or extrusion solutions can be produced both from native polymers (e.g. cellulose and cellulose derivatives) and from bio-based plastics which were obtained by means of comprehensive chemical changing of the biogenic raw materials. On the other hand, one may consider the term biopolymers to mean polymers synthesized by living creatures. These polymers are present in the form of polysaccharides, proteins, nucleic acids. Such spinning solutions can also consist of a mixture of biopolymers, such as cellulose, a solvent, a solubilising component, as well as additives necessary for the process (stabilisers, acids, alkalis) and additives that change product properties. A corresponding representation of such cellulose/amine oxide/water spinning solutions can be drawn from the publication “Structure Formation of Regenerated Cellulose Materials from NMMO-Solutions” (Prog. Polym. Sci. 26 (2001) 1473-1524).
Such polymer solutions generally have a pronounced temperature- and viscosity-sensitive behaviour, to which very close attention must be paid in all process steps, starting with the solution production, that is to say the dissolution of the polymer, through intermediate steps necessary during operation, such as conveying, distribution, filtration, heat exchanging, addition of additives and shaping. The publication “Rheology of Concentrated N-Methylmorpholine-N-Oxide Cellulose Solutions” (Polymer Science, Ser. A Vol. 39, No. 9 1997, pp 1033-1040) deals with the rheology of such polymer compounds in particular.
US 2009/304890 A1 describes a pipeline system made up of a plurality of heat exchangers, which are surrounded with a heat-transfer medium jacket for temperature regulation. Inside, the pipelines are equipped with lamellae, in order to increase the turbulence. Baffle plates can also be provided.
US 2009/165994 A1 relates to a heat exchanger with inner heat-transfer-medium conveying structures, which are provided both for heat transport and for thorough mixing. A pipe system for polymerisation of acrylamides is described in U.S. Pat. No. 4,110,521 A, which comprises heat exchangers with a temperature-control jacket and inner static mixers.
U.S. Pat. No. 5,046,548 A shows a heat exchanger with an internal double spiral, which carries the heat-transfer medium. If appropriate, an internal straight return pipeline can also be provided.
WO 2009/122143 A2 relates to a “pulse flow reactor” (PFR) with fittings, which effect an oscillating movement of the through-pumped fluid material. Means for heating the material, such as e.g. a hot water jacket, are also described in this document.
WO 2005/119154 A1 describes a system for heating suspensions with high density, which have a tendency toward laminar flow with low internal heat transfer. The system has a multiplicity of individual heat exchanger units, wherein each heat exchanger has a plurality of inner pipes. The patent publications US 2009/117218 A1, DE 10 2009 043788 A1, DE 102 41 276 A1, FR 1,383,810 A and EP 1 350 560 A1 describe further heat exchangers.
It is established in the previously mentioned publications that temperature and viscosity influences are to be taken into account in the shaping process. Corresponding investigations prove that very close attention must be paid to the shaping to form moulded products, such as staple fibres, filaments, films, mouldings and non-woven materials. The highest demands are placed on the quality of the polymer solution for producing high quality moulded products, as the polymer solution is exposed to the highest load in the shaping process.
In addition to the required quality of the polymer melt, such as temperature and viscosity uniformity, care must also be taken that on the one hand, the polymer solutions are conveyed thermally homogeneously and no thermally induced decomposition of the polymer (cellulose) and also the solvent (amine oxide) takes place in the NMMO (N-Methylmorpholine N-oxide) process for producing moulded products. It is known that autocatalytic decomposition reactions that may occur spontaneously may occur under certain conditions in the case of the previously mentioned cellulose polymer solutions. In the case of reactions of this type, it is also necessary to be able to dissipate the resulting reaction heat in as controlled a manner as possible.
Cellulose/amine oxide/water polymer solutions also have the characteristic of possibly discolouring under the action of heat. This discolouration can be so extensive that the polymer solution changes from being honey-coloured at the start of solution production to being more of a dark-brown to black over the transport path. This discolouration is caused by the thermal load on the polymer and the solvent. A strongly discoloured polymer solution leads to the end product produced at the processing location likewise taking on a dark-brown colour and therefore being unsuitable for commercial sale.
The transport of the highly viscous cellulose solution through pipes on its own leads to frictional heat being induced by the pressure resistance of the pipe (1 to 5 bar/m) and being introduced into the polymer compound. Since polymer solutions produced on a large scale often have impurities, these impurities, e.g. swelling bodies, are eliminated before the processing of the polymer solution by means of filtration. A pressure loss is generated by the filter medium, due to the filtration, which induces additional frictional heat in the polymer compound. In order to bring the polymer solution produced to the individual processing locations, the polymer solution is usually divided via angled pieces, T-pieces, Y-pieces and multiple distributors, which results in a further heat input.
Due to the displacement or conveying of the highly viscous polymer solutions by means of pumps, such as for example gear pumps, extruders, screw pumps, channel pumps, centrifugal pumps, additional frictional heat is induced and contributed to the viscosity and thermally sensitive cellulose solution.
Not only the previously mentioned plant components can contribute frictional heat (i.e. power loss of the devices), but also built-in mixers, such as static mixers for example, pipe mixers, etc., which likewise leads to frictional heat.
A system for transporting polymer solutions, starting from solution production, via pumps, filters, distribution components through to final processing devices, requires complex plant systems, so that all of the previously mentioned sources of frictional heat can be removed from the polymer solution at the location of origin, so that a temperature and viscosity uniformity is achieved for the shaping of the polymer solution, whilst maintaining the highest safety standards.